Methods and systems for characterizing a sorbent tube

ABSTRACT

Disclosed are systems and methods that include providing a vessel having an adsorbent disposed therein, the vessel having an inlet and an outlet for communicating a carrier gas through the vessel; determining a differential pressure between the inlet and the outlet for a known flow rate at the outlet; and, determining a geometric measure for the vessel based on a ratio of the known flow rate and the differential pressure. In some embodiments, the methods and systems can include a viscosity in the ratio, determining a flow rate at ambient pressure using a factor based on a ratio of a gas pressure at the output and ambient pressure, and/or, determining a flow rate at ambient temperature using a factor based on a ratio of a temperature at the output and ambient temperature.

PRIOR APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/072,371, filed Mar. 4, 2005, now U.S. Pat. No. 7,168,296.

FIELD OF THE INVENTION

The present invention relates to systems and methods for measuring theintegrity of a sorbent tube that can be used, for example, inchromatographic systems. More specifically, the invention relates tomethods and systems that include a measurement of a geometriccharacteristic of the sorbent tube.

BACKGROUND OF THE INVENTION

Sorbent tubes have many different applications, and include a packingmaterial that can be used for adsorption. One application for sorbenttubes includes gas chromatography. Gas chromatography is essentially aphysical method of separation in which constituents of a test sample ina carrier gas are adsorbed or absorbed and then desorbed by a stationaryphase material in a column. A pulse of the sample is introduced into asteady flow of carrier gas, which carries the sample into achromatographic column. The inside of the column is lined with a liquid,and interactions between this liquid and the various components of thesample—which differ based upon differences among partition coefficientsof the elements—cause the sample to be separated into the respectiveelements. At the end of the column, the individual components are moreor less separated in time. Detection of the gas provides a time-scaledpattern, typically called a chromatogram, that, by calibration orcomparison with known samples, indicates the constituents, and thespecific concentrations thereof, which are present in the test sample.An example of the process by which this occurs is described in U.S. Pat.No. 5,545,252 to Hinshaw.

One common application of chromatographic analysis is the use of thermaldesorption units to determine the constituents of a particularenvironment. For example, it is often desired to detect the amount ofvolatile organic compounds (VOCs) present in a certain sample of air.One way of doing this is by first transporting a tube packed with anadsorbent material into the environment to be tested, and allowing theVOCs in the air to migrate into the tube through natural diffusion,typically termed “diffusive” or “passive sampling.” Alternatively, theVOCs may be collected by drawing a sample of gas (typically ambient air)through such a tube using a small vacuum pump, commonly referred to as“pumped sampling.” In each case, the analytes to be measured (i.e., theVOCs) are retained by and concentrated on the adsorbent as the airpasses through the tube. As is briefly described in U.S. Pat. No.6,649,129 to Neal, once the VOCs are first collected in this fashion,and then, the tube is subsequently heated in a thermal desorptioninstrument, and a flow of inert gas, such as Helium or Nitrogen, isapplied to the tube to sweep the VOCs out of the tube and into thechromatographic column for separation and analysis.

Regardless of what particular application is being used, it is alsooften desired to pre-concentrate the analytes in the sample, andoccasionally, remove moisture therefrom, prior to introducing the sampleinto the chromatographic column. Accordingly, as disclosed in U.S. Pat.Nos. 5,792,423 and 6,395,560 to Markelov, these systems will typicallyinclude some kind of “trap” for this purpose, which retains the analytesas they are carried through the trap, and which are later released fromthe trap, usually by heating, and swept into the chromatographic column.One example is an adsorbent trap, which typically includes a tube packedwith a suitable adsorbent material, which adsorbs the analytes as thesample gas first passes through the tube, and from which the analytesare subsequently desorbed into the chromatographic column, such as thearrangements disclosed in U.S. Pat. No. 5,932,482 to Markelov and U.S.Pat. No. 6,652,625 to Tipler. flow rate at a tube outlet. Although theillustrated embodiments include applications directed to gaschromatography systems and methods, it can be understood that thepresent teachings can be equally applied to other sorbent tubeapplications.

In an embodiment, the present teachings include methods that includeproviding a vessel having an adsorbent disposed therein, the vesselhaving an inlet and an outlet for communicating a carrier gas throughthe vessel, determining a differential pressure between the inlet andthe outlet for a known flow rate at the outlet; and, determining ageometric measure for the vessel based on a ratio of the known flow rateand the differential pressure. In determining a differential pressure,the methods can include providing at least one sensor to providemeasurements related to the differential pressure. Determining ageometric measure can also include factoring a viscosity of the carriergas in the ratio. In embodiments, the vessel may include an adsorbenttrap or a sample tube. The methods can further include one or more ofdetermining a flow rate at ambient pressure using a factor based on aratio of a gas pressure at the output and ambient pressure, and,determining a flow rate at ambient temperature using a factor based on aratio of a temperature at the output and ambient temperature.

In some embodiments, the present teachings relate to a chromatographicmethod that includes providing a vessel in a carrier gas flow path to achromatographic column, where the vessel includes a gas inlet, a gasoutlet, and, an adsorbent disposed in the vessel. The chromatographicmethods also include measuring a differential pressure along the lengthof the adsorbent, determining a flow rate at the outlet, and,determining a unit flow per unit pressure based on a ratio of the flowrate and the differential pressure. In some embodiments, the methods caninclude adjusting the unit flow per unit pressure based on a viscosityof the gas. In embodiments, the chromatographic methods includedetermining (i) a flow rate at ambient pressure using a factor based ona ratio of a gas pressure at the output and ambient pressure, and/or,(ii) a flow rate at ambient temperature using a factor based on a ratioof a temperature at the output and ambient temperature.

In an embodiment, the present teachings thus comprise a chromatographicsystem, including a carrier gas inlet for supplying carrier gas, achromatographic column for receiving the carrier gas, a flow paththrough which the carrier gas is communicated from the carrier gas inletto the chromatographic column, a vessel disposed in the flow path, thevessel having an adsorbent disposed therein, wherein the vessel includesan inlet and an outlet for communicating the carrier gas through thevessel, and, at least one sensor in communication with said flow path,the at least one sensor providing at least one

However, one problem that exists in these various systems is that thesorbent tubes serving as the traps, as well as the sorbent tube servingas initial sampling tubes when dealing with applications involving thetraditional thermal desorption units discussed above, is that theintegrity of these tubes is sometimes compromised. Several causes ofthis problem are illustrated in FIGS. 1A-B. For example, in order forthermal desorption to work successfully, the adsorbent 10 must beproperly packed inside the tube 12. However, sometimes this does notoccur, and voids 14 are formed in the adsorbent, as shown in FIG. 1A.These voids will channel some of the gas flow, thereby degrading theadsorption and desorption efficiency of the adsorbent packing.

Similarly, occasionally, the adsorbent becomes damaged as a result ofimproper packing or thermal shock, thereby producing small fragments(fines) 16 that occlude the interstices between the packing particles,as shown in FIG. 1B. As a result, the flow of gas is partially blockedduring adsorption and desorption, again degrading the efficiency of thesorbent tube.

SUMMARY OF THE INVENTION

Accordingly, the present teachings provide systems and methods fordetermining a geometric measure of a sorbent tube (“tube”) based ondetermining a differential pressure across the tube and relating thesame to a measurement for determining a differential pressure along theadsorbent material which when related to a flow rate at the vesseloutlet and a viscosity of the carrier gas, provides a unit flow per unitpressure for said carrier gas in said vessel. The sensor(s) can includeat least one sensor for measuring a pressure at the vessel inlet and apressure at the vessel outlet.

In an embodiment, the invention comprises a method of performing achromatographic analysis, including providing a carrier gas inlet forsupplying carrier gas, providing a chromatographic column for receivingthe carrier gas, providing a vessel having an adsorbent disposed thereinfor adsorbing and desorbing analytes to be measured, the vessel havingan inlet and an outlet for communicating carrier gas through the vessel,communicating carrier gas from the carrier gas inlet, into the vesselthrough the vessel inlet, and out of the vessel through the vesseloutlet, and determining the impedance of the gas communicated throughthe vessel.

In one embodiment, the invention comprises a chromatographic system,including a carrier gas inlet for supplying carrier gas, achromatographic column for receiving the carrier gas, a flow paththrough which the carrier gas is communicated from the carrier gas inletto the chromatographic column, a vessel disposed in the flow path, thevessel having an adsorbent disposed therein for adsorbing and desorbinganalytes to be measured, wherein the vessel includes an inlet and anoutlet for communicating the carrier gas through the vessel, and atleast one sensor in communication with the flow path adjacent at leastone of the vessel inlet and the vessel outlet for measuring at least onecharacteristic of the gas communicated through the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an exposed side view of a sorbent tube illustrating voids inan adsorbant;

FIG. 1B is an exposed side view of a sorbent tube illustratingocclusions in interstices between packing particles of an adsorbant;

FIG. 2A is a schematic view of an example chromatographic system thatmay be used during a trap load stage;

FIG. 2B is a schematic view of the chromatographic system of FIG. 2Aduring a trap desorb stage;

FIG. 3 is a schematic view showing additional detail of the system ofFIGS. 2A-B during a sample vessel dry purge stage;

FIG. 4 is a schematic view of the system of FIG. 3 during a samplevessel desorption and trap load/purge stage;

FIG. 5 is a schematic view of the system of FIG. 3 during a trapdesorption stage;

FIG. 6A is a schematic view showing additional detail of the measurementof the gas flow through the vessels of FIG. 3;

FIG. 6B is a schematic view showing additional detail of the measurementof the gas flow through the vessels of FIG. 3;

FIG. 6C is a schematic view showing additional detail of the measurementof the gas flow through the vessels of FIG. 3;

FIG. 6D is a schematic view showing additional detail of the measurementof the gas flow through the vessels of FIG. 3;

FIG. 6E is a schematic view showing additional detail of the measurementof the gas flow through the vessels of FIG. 3;

FIG. 6F is a schematic view showing additional detail of the measurementof the gas flow through the vessels of FIG. 3; and,

FIG. 7 is a schematic view showing additional detail of gas flow throughthe vessels of FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS

The basic components of one embodiment of a chromatographic system thatverifies the integrity of a sorbent tube in accordance with the presentteachings are illustrated in FIGS. 2A-B. As used in the description, theterms “top,” “bottom,” “above,” “below,” “over,” “under,” “above,”“beneath,” “on top,” “underneath,” “up,” “down,” “upper,” “lower,”“front,” “rear,” “back,” “forward” and “backward” refer to the objectsreferenced when in the orientation illustrated in the drawings, whichorientation is not necessary for achieving the objects of the invention.

The chromatographic system typically includes a sample container, suchas sample tube 20, canister 22, or headspace vial 24, from which asample containing the analytes to be measured is supplied to the system,though, in some applications, a sample of the surrounding atmosphere ispumped directly into the system via an inlet 26. The sample is usuallyextracted from the sample container or placed in a flow path of carriergas, and the gas carries the sample through the system, as is furtherdescribed below. As shown in FIG. 2A, the sample is typically firstcarried through an analyte pre-concentrator, which, in some embodiments,comprises a vessel 30, such as a tube, with an adsorbent 32 disposedtherein, often referred to as an adsorbent trap. As shown in FIG. 2B,after the adsorbent 32 adsorbs the analytes and the remainder of the gasmixture passes through the vessel 30 and vents from the system, thevessel 30 is heated by a heating element 34 and the analytes aredesorbed into a chromatographic column 40.

A system utilizing a thermal desorption unit for testing samplescollected in sample tubes is shown in detail in FIG. 3. In thesesystems, a vessel, such as the sorbent tube 100, is transported to alocation from which a sample is to be collected. Depending on theparticular application, the sample may be collected in the vessel 100 indifferent ways, such as, for example, by pumping the sample into thevessel 100, or, in a particular environment to be tested, by allowingcomponents in the air to simply migrate into the vessel 100 throughnatural diffusion. The vessel 100 is then transported to the thermaldesorption device, where it may be placed in fluid communication with aseparate adsorbent trap 102 for further concentration of the analytesprior to desorption into the chromatographic column 104.

The operation of this system is illustrated stepwise in FIGS. 3-5. Asample vessel dry purge step is illustrated in FIG. 3. As shown therein,a rotary valve 106 is positioned such that a carrier gas inlet 108, thesample vessel 100, and the trap 102 are in fluid communication. Carriergas flows from the inlet 108 to the first end 120 of the sample vessel100. The gas enters the vessel 100 through the first end 120, flowsthrough the adsorbent 110, flows out of the vessel 100 via the inlet 130of conduit 132, and exits through the vent 112, indicated by arrows A.In this way, moisture in the vessel 100 is purged therefrom. In certainembodiments, a valve 160 is also provided in the event a split flow isdesired.

A sample vessel desorption and trap load/purge step is illustrated inFIG. 4. As shown therein, a diverter valve 114 directs carrier gas fromthe inlet 108 to the second end 122 of the sample vessel 100. Thecarrier gas flows into the vessel 100 via the outlet 134 of the conduit136, through the adsorbent 110, out through the first end 120 of thevessel 100, and into the trap 102, indicated by arrows B. In this way,the carrier gas, sweeps up the analytes as they are desorbed from theadsorbent 110 and carries them into the trap 102. The adsorbent 140 inthe trap 102 adsorbs the analytes, and the carrier gas flows out of thetrap 102 via the inlet 150 of the conduit 152 and exits through the vent142, also indicated by arrows B.

A trap desorption step is illustrated in FIG. 5. As shown therein, therotary valve 106 is rotated and the valve 144 is opened. Carrier gasflows from the inlet 146, through the conduit 148, and into the trap 102via the outlet 154. The gas then flows through the adsorbent 140,sweeping up the analytes as they are desorbed, and carries them out ofthe trap 102 and into the chromatographic column 104, indicated byarrows C.

Because the sample tube 100 and the trap 102 each comprise a vesselpacked with an adsorbent, either one of them, or both, could haveundesired formation of channels or fines, as previously discussed. Thepresence of these channels and/or fines will alter the pneumaticimpedance of the trap, and therefore, the system described herein checksfor alterations in this impedance in order to determine whether theresults of any chromatographic analysis have been degraded as a resultof the integrity of the tube and/or trap being compromised.

This impedance measurement can be achieved by elevating the pressures inthe internal flow paths of the system, as can be seen by referencingFIG. 7 and measuring characteristics of the gas flowing through thevessel 100 and/or 102 and comparing the measured characteristics withexpected values of such characteristics. Expected values of somecharacteristics are provided herein as examples, and it can beunderstood that other characteristics and/or ways of computing suchexpected characteristics can be varied without departing from the scopeof the present teachings.

For example, in one embodiment, the sorbent tubes 100, 102 can beunderstood to be the geometric equivalent of a short, packed, gaschromatographic column, and therefore, the permeability of the sorbenttube can be represented according to the following equation:

$\begin{matrix}{B^{o} = \frac{2{\cdot \eta \cdot ɛ \cdot L \cdot p_{o} \cdot u_{o}}}{\left( {p_{i}^{2} - p_{o}^{2}} \right)}} & (1)\end{matrix}$Where:

-   -   B^(o) is the specific permeability coefficient    -   η is the viscosity of the gas flowing through the tube    -   ε is the inter-particle porosity of the adsorbent packing    -   L is the length of the tube    -   p_(i) is the absolute gas pressure at the tube inlet    -   p_(o) is the absolute gas pressure at the tube outlet    -   u_(o) is the gas velocity at the tube outlet at pressure p_(o)        The inter-particle porosity of the packing ε may, in turn, be        represented according to the following equation:

$\begin{matrix}{ɛ = \frac{V_{i}}{V_{c}}} & (2)\end{matrix}$Where:

-   -   V_(i) is the inter-particle volume    -   V_(c) is the volume of the empty tube        The gas velocity at the tube outlet u_(o) can be represented by        rearranging Equation 1 as follows:

$\begin{matrix}{u_{o} = \frac{B^{o} \cdot \left( {p_{i} + p_{o}} \right) \cdot \left( {p_{i} - p_{o}} \right)}{2 \cdot \eta \cdot ɛ \cdot L \cdot p_{o}}} & (3)\end{matrix}$Because p_(i)≈p_(o), Equation 3 may be further simplified as follows:

$\begin{matrix}{u_{o} = \frac{B^{o} \cdot \left( {p_{i} - p_{o}} \right)}{\eta \cdot ɛ \cdot L}} & (4)\end{matrix}$Accordingly, it is established that the velocity through the sorbenttube is proportional to the pressure difference across it.

The volumetric flow rate at the tube outlet can be represented accordingto the following equation:F _(o) =u _(o) ·A _(i)  (5)Where A_(i) is the inter-particle area. Theinter-particle-to-open-tube-area ratio should be the same as thevolumetric ratio (which, according to Equation 2, is equal to thepacking porosity), as shown in the following equation:

$\begin{matrix}{\frac{A_{i}}{A_{c}} = {\frac{V_{i}}{V_{c}} = ɛ}} & (6)\end{matrix}$Where A_(c) is the cross sectional area of the empty tube. Therefore,the inter-particle area A_(i) can be represented as follows:A _(i) =A _(c)·ε  (7)Thus, the substitution of Equation 7 into Equation 5 yields thefollowing representation of the volumetric flow rate:F _(o) =u _(o) ·A _(c)·ε  (8)Because the cross-sectional area of a cylindrical tube A_(c) is equal to

$\frac{\pi \cdot d_{c}^{2}}{4},$Equation 8 becomes:

$\begin{matrix}{F_{o} = \frac{\pi \cdot u_{o} \cdot d_{c}^{2} \cdot ɛ}{4}} & (9)\end{matrix}$

Substituting the gas velocity at the tube outlet u_(o) of Equation 4into Equation 9 yields the following representation of the volumetricflow rate:

$\begin{matrix}{F_{o} = \frac{B^{o} \cdot \pi \cdot d_{c}^{2} \cdot \left( {p_{i} - p_{o}} \right)}{4 \cdot \eta \cdot L}} & (10)\end{matrix}$For a given tube at constant temperature, B^(o), d_(c), η and L shouldall be constant, which can collectively be represented by the constantk, yielding the following equation:

$\begin{matrix}{F_{o} = \frac{k \cdot \left( {p_{i} - p_{o}} \right)}{\eta}} & (11)\end{matrix}$Where k is a constant and equal to

$\frac{B^{o} \cdot \pi \cdot d_{c}^{2}}{4 \cdot L}.$Accordingly, expressing equation 11 in terms of k allows for acharacterization of the tube in terms of unit flow per unit pressuredrop for a given temperature. k can thus be understood to be a geometricmeasure of the tube rather than the ambient conditions to which the tubeis exposed. With further reference to Equation 11, once a differentialpressure of the adsorbent material is obtained using, for example, oneor more sensors, such differential pressure can be related to the flowrate at the output of the tube to characterize the tube based on a unitflow per unit pressure drop.

The value of F_(o) in Equation 11 would represent the flow rate atpressure p_(o). Normally, flow rates are expressed assuming ambientconditions (e.g. Standard Ambient Temperature and Pressure at 25° C. and100 kPa). Accordingly, a correction may be made in accordance with thefollowing equation:

$\begin{matrix}{F_{a} = {F_{o} \cdot \frac{p_{o}}{p_{a}} \cdot \frac{T_{a}}{T_{o}}}} & (12)\end{matrix}$Where:

-   -   F_(a) is the equivalent flow rate at ambient pressure and        temperature    -   p_(a) is the ambient absolute pressure    -   T_(a) is the ambient absolute temperature    -   T_(o) is the absolute temperature at the tube outlet        Accordingly, Equation 11 becomes the following equation:

$\begin{matrix}{F_{a} = {\frac{k \cdot \left( {p_{i} - p_{o}} \right)}{\eta} \cdot \frac{p_{o}}{p_{a}} \cdot \frac{T_{a}}{T_{o}}}} & (13)\end{matrix}$

As provided herein, the constant k is a geometric measure of the tube asit is based on the geometry of the particular tube (L and d_(c)) andspecific permeability of the packing therein (B^(o)). Therefore,constant k may be called the “permeability factor” of a sorbent tube.Once the value of k is known for a given tube, the flow rate for a givenpressure drop across the tube, or the pressure drop for a given flowrate, may be calculated according to Equation 13.

Accordingly, there are several ways that an alteration in the impedanceof the tubes 100, 102 can be determined, some of which are illustratedin FIGS. 6A-F. For example, in some embodiments, this determination ismade by measuring the pressure drop across the tube while carrier gasflows through the tube at a known flow rate. Typical flow rates employedfor this purpose fall in the range of about 20 mL/min to about 200mL/min, and in some embodiments, more specifically around 50 mL/min,although such example flow rates are provided for illustration and notlimitation.

As illustrated in FIG. 6A, in some embodiments, a flow controller 200,such as an electronic mass flow controller, is placed in the flow pathof the carrier gas in order to control the flow rate of the gas, and adifferential pressure transducer 202 is placed in communication with theflow path to provide pressure measurements to measure the pressure dropacross the tube 102, which in some embodiments can include sensors atthe inlet or outlet of the tube to derive the pressure drop bysubtraction. A forward pressure regulator 230 can be used to delivercarrier gas at a known and consistent pressure to the tube. A pressuretransducer 206 can be included in the circuit either upstream (as shown)or downstream of the tube to provide the absolute or gauge pressure ofthe gas within the tube to allow calculation of the gas flow rate at thetube exit at the applied pressure. In instances where the measurement isbeing made at a temperature other than ambient, a temperature sensor 233held proximal to the tube is used to calculate the gas flow rate at thetube exit at the temperature applied there. While in certainembodiments, the flow controller 200 is placed downstream of the vessel102, in some embodiments, as illustrated in FIG. 6B, a flow controller204 is placed upstream of the vessel 102. As also shown therein, in someembodiments, a first transducer 206 is placed approximate the vesselinlet, and a second transducer 208 is placed approximate the vesseloutlet, to provide measurements associated with the absolute or gaugepressure at the that point and the pressure difference across thevessel, although it can be understood that the positioning of suchtransducers 206, 208 may vary based on the transducer type, theapplication, and other design choices. As illustrated in FIG. 6C, instill other embodiments, only pressure transducer 206 is used, and avalve 220 is provided to vent the gas exiting the vessel outlet to theatmosphere at atmospheric pressure. A single gauge pressure transducer206 is used to provide a measure of both the pressure inside the tubeand the pressure drop across it.

In some embodiments, however, the pressure drop is known, controlled,and applied, and the flow rate is instead detected in order to measurethe impedance of the vessel 102. For example, as illustrated in FIG. 6D,in some of these embodiments, a forward pressure regulator 230 is placedin the flow path between the carrier gas inlet and the vessel inlet, anda back pressure regulator 232 is placed in the flow path between thevessel outlet and the chromatographic column in order to establish thepressure drop across the vessel 102. A flow sensor 240 is thenpositioned at a convenient point within in the flow path in order tomeasure the flow rate of the gas. In embodiments, as shown in FIG. 6E,only the forward pressure regulator 230 is employed, and the valve 220,which is positioned between the vessel outlet and the chromatographiccolumn, vents the gas to atmosphere at atmospheric pressure. As shown inFIG. 6F, in some of these embodiments that use pressure regulators toestablish the pressure drop, a differential pressure transducer 202 isplaced in communication with the flow path adjacent the vessel inlet andoutlet in order to verify the pressure drop established by the pressureregulator 230 and the regulator 232 or valve 220.

In these ways, changes in the pneumatic impedance of sorbent tubes canbe detected as part of the chromatographic analysis, both for tubesemployed as adsorbent traps, where the aging effects on the integrity ofthe packing is of particular concern, and for sample tubes used tocollect and supply the sample for analysis, where variations from tubeto tube can otherwise prove problematic. In embodiments, the specificmethod of measurement is standardized so that meaningful results can beobtained so that the tube impedance can be tracked over the life of thetube. Further, in embodiments, the tube is kept at ambient temperature(typically in the range of about 20 to about 25 degrees Celsius) duringmeasurement in order to reduce error.

It should be understood that the foregoing is illustrative and notlimiting, and that obvious modifications may be made by those skilled inthe art without departing from the spirit of the invention. For example,throughout the entirety of the present disclosure, use of the articles“a” or “an” to modify a noun can be understood to be used forconvenience and to include one, or more than one of the modified noun,unless otherwise specifically stated. Further, although the illustratedexamples include a sorbent “tube”, it can be understood that the presentteachings are not limited to a vessel having a particular cross section,and the methods and systems are applicable to different size and shapedvessels having an adsorbent material contained and/or disposed therein.Accordingly, reference should be made primarily to the accompanyingclaims, rather than the foregoing specification, to determine the scopeof the invention.

1. A chromatographic method, comprising: providing a vessel in a carriergas flow path to a chromatographic column, the vessel comprising: a gasinlet, a gas outlet, and, an adsorbent disposed in the vessel, measuringa differential pressure along the length of the adsorbent, determining aflow rate at the outlet, and, determining a unit flow per unit pressurebased on a ratio of the flow rate and the differential pressure.
 2. Themethod of claim 1, further comprising adjusting the unit flow per unitpressure based on a viscosity of the gas.
 3. The method of claim 1,further comprising at least one of: determining a flow rate at ambientpressure using a factor based on a ratio of a gas pressure at the outputand ambient pressure, and, determining a flow rate at ambienttemperature using a factor based on a ratio of a temperature at theoutput and ambient temperature.
 4. The method of claim 1, furthercomprising heating the vessel to desorb analytes from the adsorbent. 5.The method of claim 1, wherein the vessel comprises an adsorbent trap.6. The method claims 1, wherein the vessel comprises a sample tube. 7.The method of claim 6, further comprising: providing a second vessel inthe carrier gas flow path to the chromatographic column, the secondvessel comprising: a gas inlet, a gas outlet, and, an adsorbent disposedin the second vessel, measuring a differential pressure along the lengthof the adsorbent disposed in the second vessel, determining a flow rateat the outlet of the second vessel, and, determining a second unit flowper unit pressure based on a ratio of the flow rate at the outlet of thesecond vessel and the differential pressure along the length of theadsorbent disposed in the second vessel.
 8. A chromatographic method,comprising: providing a vessel in a carrier gas flow path to achromatographic column, the vessel comprising: a gas inlet, a gasoutlet, and, an adsorbent disposed in the vessel, measuring adifferential pressure along the length of the adsorbent, and relatingthe differential pressure to a flow rate at the outlet to determine aunit flow per unit pressure based on a ratio of the flow rate and thedifferential pressure.
 9. The method of claim 8, further comprisingadjusting the unit flow per unit pressure based on a viscosity of thegas.
 10. The method of claim 8, further comprising establishing the flowrate at the outlet by controlling the flow of the gas.
 11. The method ofclaim 8, wherein the vessel comprises an adsorbent trap.
 12. The methodclaims 8, wherein the vessel comprises a sample tube.
 13. The method ofclaim 12, further comprising: providing a second vessel in the carriergas flow path to the chromatographic column, the second vesselcomprising: a gas inlet, a gas outlet, and, an adsorbent disposed in thesecond vessel, measuring a differential pressure along the length of theadsorbent disposed in the second vessel, and, relating the differentialpressure along the length of the adsorbent disposed in the second vesselto a flow rate at the outlet of the second vessel to determine a secondunit flow per unit pressure based on a ratio of the flow rate at theoutlet of the second vessel and the differential pressure along thelength of the adsorbent disposed in the second vessel.